The present invention relates to a magneto-optical recording medium, method for reproducing it and a reproducing apparatus therefor; in more detail it relates to a magneto-optical recording medium, method for reproducing it and a reproducing apparatus therefor suitable for high density recording whereby minute recording magnetic domains smaller than the reproducing light spot can be magnified and reproduced.
Since in magneto-optical recording media the recorded information can be re-recorded, storage capacity is large and reliability is high, they have started to be implemented as external memories etc. for computers. However, with increases in the amount of information and increased compactness required of the apparatus, further demands are being made on high-density recording and reproducing techniques. To record information on a magneto-optical recording medium, the magnetic field modulation method is employed, wherein a magnetic field of polarity corresponding to the recording signal is applied to a region of the magneto-optical recording medium which has been raised in temperature by directing a laser beam onto it. With this method, over-write recording is possible and furthermore high-density recording with for example a shortest mark length of 0.15 xcexcm can be achieved. Furthermore overwrite recording is possible and has been implemented with an optical modulation recording system in which recording is performed under a constant applied magnetic field, using a light-beam whose power is modulated in accordance with the recording signal.
However, the optical reproduction resolution, which is determined by the spot radius of the reproducing light-beam, presents a problem when recording marks which are recorded at high density are to be reproduced. For example, it is not possible to identify and reproduce minute marks of magnetic domain length 0.2 xcexcm using a reproducing light beam of spot diameter 1 xcexcm. As one approach to eliminating restrictions on reproduction resolution resulting from the optical spot radius of the reproducing light beam, a Magnetically Induced Super Resolution (MSR) technique has been proposed as described in for example Journal of Magnetic Society of Japan, Vol. 17 Supplement No. S1, pp. 201 (1993). In this technique, the effective spot radius that contributes to the reproduction signal is reduced by generating a magnetic mask within the spot by using a temperature distribution generated in the magnetic film within the reproducing light beam spot when the reproducing light beam is directed onto the magneto-optical recording medium. By using this technique, the reproduction resolution can be raised without actually decreasing the spot radius of the reproducing light beam. However, with this technique, the spot radius is effectively reduced by means of the magnetic mask, so the amount of light contributing to the reproduction output is lowered, and so the reproduction C/N is correspondingly lowered. As a result, it is difficult to obtain a satisfactory C/N.
Japanese Patent Laid-Open Publication No. 1-143041 discloses a method of reproducing a magneto-optical recording medium in which reproduction is performed by magnifying the recording magnetic domain of a first magnetic film using a magneto-optical recording medium comprising a first magnetic film, a second magnetic film and a third magnetic film that are mutually magnetically coupled at room temperature and in which, if the Curie temperatures of the first magnetic film, second magnetic film and third magnetic film are assumed to be TC1, TC2 and TC3, then TC2 greater than room temperature while TC2 less than TC1 and TC3, the coercivity HC1 of the first magnetic film being sufficiently small in the vicinity of the Curie temperature TC2 of the second magnetic film and the coercivity HC3 of the third magnetic film being sufficiently greater than the required magnetic field in the temperature range from room temperature up to a required temperature TPB higher than TC2. In this method, utilising the rise in temperature of the medium on illumination by the reproducing light beam, magnetic coupling of the first and third magnetic films is cut off and in this condition the magnetic domain of the first magnetic film is magnified by a demagnetising field acting on the recording magnetic domain and an externally applied magnetic field. It should be noted that, in this method, a second magnetic film is employed whose Curie temperature is set lower than the temperature of the reproduction portion during reproduction, but in the present invention a magnetic film having such a magnetic characteristic is not employed.
Japanese Patent Laid-Open Publication No. 6-295479 discloses a magneto-optical recording medium having a first magnetic layer in which magnetization direction changes from an in-plane direction to a perpendicular direction at a certain transition temperature above room temperature and a second magnetic layer (recording layer) consisting of perpendicularly magnetizable film. The transition temperature of the first magnetic layer becomes higher in the film thickness direction from the side where the light enters, either continuously or stepwise. The first magnetic layer of this magneto-optical recording medium is constituted by a reproducing layer, a first intermediate layer and a second intermediate layer; the temperatures at which the transition takes place from in-plane magnetization to perpendicular magnetization are set to be higher in the order: reproducing layer, first intermediate layer and second intermediate layer, so on information reproduction, due to the relationship of the transition temperatures of the respective layers and the temperature distribution within the reproducing light beam spot, the magnetic domain of the recording layer is magnified and is transferred to the reproducing layer. However, in this publication, there is no detailed description regarding the thickness of the intermediate layers; the total film thickness of the intermediate layers of the magneto-optical recording medium used in an embodiment is 10 nm.
An object of the present invention is to provide a magneto-optical recording medium and a signal reproducing method and reproducing apparatus therefor whereby a reproduction signal with satisfactory C/N can be obtained even when minute magnetic domains are recorded.
According to a first aspect of the present invention, there is provided a magneto-optical recording medium comprising, at least a magneto-optical recording layer on which information is recorded, a first auxiliary magnetic layer and a second auxiliary magnetic layer in which, when irradiated with a reproducing light beam, a recording magnetic domain recorded in the magneto-optical recording layer is magnified and transferred (transmitted) to the second auxiliary magnetic layer through the first auxiliary magnetic layer and information is reproduced from this magnetic domain of the second auxiliary magnetic layer which has thus been magnified and transferred, characterized in that,
the thickness of the first auxiliary magnetic layer is not less than the thickness of the magnetic wall of this first auxiliary magnetic layer.
According to a second aspect of the present invention, there is provided a magneto-optical recording medium comprising at least a magneto-optical recording layer on which information is recorded, a first auxiliary magnetic layer and a second auxiliary magnetic layer wherein, when irradiated with a reproducing light beam, a recording magnetic domain recorded in the magneto-optical recording layer is magnified and transferred to the second auxiliary magnetic layer through the first auxiliary magnetic layer and information is reproduced from this magnetic domain of the second auxiliary magnetic layer which has thus been magnified and transferred, characterized in that,
the thickness of the first auxiliary magnetic layer exceeds 10 nm.
An example of major parts of a magneto-optical recording medium according to the present invention is illustrated diagrammatically in FIGS. 2A and 2B. The magneto-optical recording medium has a construction in which a first auxiliary magnetic layer (film) 5 and a second auxiliary magnetic layer (film) 4 are successively provided on a magneto-optical recording layer (film) 6. As shown in FIG. 3, first auxiliary magnetic layer 5 and second auxiliary magnetic layer 4 have a magnetic characteristic such that they constitute in-plane magnetized layers from room temperature up to a certain temperature (critical temperature) TCR above room temperature, but constitute perpendicularly magnetizable layers above this TCR. Magneto-optical recording layer 6 shows perpendicular magnetization in a wide temperature range including room temperature. Taking the Curie temperatures of magneto-optical recording layer 6, first auxiliary magnetic layer 5 and second auxiliary magnetic layer 4 as being TC0,TC1 and TC2, and the critical temperatures of the first auxiliary magnetic layer and second auxiliary magnetic layer as being TCR1 and TCR2, the magnetic characteristics in this magneto-optical recording medium satisfy the relationships: room temperature less than TCR2 less than TCR1 less than TC0, TC1, TC2.
The principle of reproduction of a magneto-optical recording medium having the construction shown in FIGS. 2A and 2B will now be described. FIG. 2A shows the magnetization state of the respective layers prior to reproduction; it is assumed that recording magnetic domain 22 in magneto-optical recording layer 6 has been previously recorded by a magnetic field modulation system or an optical modulation recording system. When a reproducing light beam of appropriate power such that the maximum temperature attained by the magnetic layer is a desired temperature less than TC0 is directed onto this magneto-optical recording medium, as shown in FIG. 2B, in regions where the temperature in the first auxiliary magnetic layer 5 is TCR1 or more, recorded magnetic domain 22 of magneto-optical recording layer 6 is transferred, producing magnetic domain 21. In this process, as will be described, it is desirable that the transfer (transmission) to first auxiliary magnetic layer 5 should take place in such a way that the size of magnetic domain 21 is less than the size of recording magnetic domain 22 of magneto-optical recording layer 6 i.e. in such a way that recording magnetic domain 22 is reduced. Next, magnetic domain 21 that was transferred to first auxiliary magnetic layer 5 is transferred to second auxiliary magnetic layer 4 to constitute a magnetic domain 23.
The top of FIG. 6 shows the temperature distribution produced when a magneto-optical recording medium of the construction shown in FIG. 2B is heated by a reproduction laser spot (LS); the middle of FIG. 6 shows the temperature distribution of the magneto-optical recording medium with respect to the laser spot (LS) seen from above the second auxiliary magnetic layer. In this magneto-optical recording medium, the critical temperatures of the first and second auxiliary magnetic layers are set such that TCR2 less than TCR1, so the temperature region in which TCR2 is exceeded i.e. the region of the second auxiliary magnetic layer in which a perpendicular magnetization state can be obtained, is larger than the temperature region in which TCR1 is exceeded i.e. the region of the first auxiliary magnetic layer in which a perpendicular magnetization state can be obtained. Consequently, magnetic domain 23 which is transferred into second auxiliary magnetic layer 4 is magnified, compared with the size of magnetic domain 21, due to the exchange coupling force from the transfer magnetic domain of first auxiliary magnetic layer 5 and the perpendicular magnetic anisotropy of the second auxiliary magnetic layer. Since this magnified magnetic domain 23 is larger than the recording magnetic domain 22 of magneto-optical recording layer 6 the reproduction signal that is detected by means of the magneto-optical effect (Kerr effect) is amplified from that which would have been detected from a magnetic domain of the same size as recording magnetic domain 22, making possible reproduction with high C/N. That is, although in reproduction using ordinary Magnetically Induced Super Resolution, the reproduction signal amplitude from the minute magnetic domains is very small, by using the present magneto-optical recording medium, even though a signal is reproduced from a minute magnetic domain, an amplified reproduction signal amplitude can be obtained.
In a magneto-optical recording medium according to the present invention, it is desirable that the size of a magnetic domain 21 that is transferred into first auxiliary magnetic layer 5 should be smaller than that of recording magnetic domain 22 of magneto-optical recording layer 6. That is, when recording magnetic domain 22 of magneto-optical recording layer 6 is transferred as magnetic domain 21 of first auxiliary magnetic layer 5, it is desirable that the magnetic domain should be reduced. The reasons for this will now be described.
If the size of the magnetic domain 21 (magnetized in the ↑ direction) that is transferred to first auxiliary magnetic layer 5 is the same as or greater than the size of magnetic domain 22, magnetic domain 21 is magnetically affected by magnetic domain S having magnetization in the ↓ direction and adjoining recording magnetic domain 22, with the result that magnetic domain 21 becomes unstable. Since magnetic domain 21 that is transferred to first auxiliary magnetic layer 5 is required to have the function of transferring the magnetization information of recording magnetic domain 22 to second auxiliary magnetic layer 4 having the function of magnifying the magnetic domain, it has to be magnetically stable. Consequently, by effecting transfer from a recording magnetic domain 22 to first auxiliary magnetic layer 5 with the magnetic domain being reduced in size, the effect from magnetic domain S adjoining recording magnetic domain 22 on magnetic domain 21 of first auxiliary magnetic layer 5 can be reduced, enabling the magnetization of magnetic domain 21 of first auxiliary magnetic layer 5 to be stabilised. In particular, since the magneto-optical recording medium is usually reproduced while rotating, the magnetic domains of magneto-optical recording layer 6 move successively past the reproducing light beam spot whilst the magneto-optical recording medium is being rotated as shown in FIGS. 29A and B. Furthermore, the temperature region of first auxiliary magnetic layer 5 exceeding TCR1 exists at a fixed position with respect to the reproducing light beam spot. If the temperature region of first auxiliary magnetic layer 5 exceeding TCR1 is of the same size as recording magnetic domain 22, the time for which only one recording magnetic domain 21 is in this temperature region during movement is only an instant; the rest of the time, in this temperature region there will be found part of one recording magnetic domain 21 and part of an in-plane magnetized recording magnetic domain adjacent to it. It is therefore very difficult to reproduce from the temperature region of first auxiliary magnetic layer 5 exceeding TCR1 magnetic information of a single recording magnetic domain only. However, if the temperature region of first auxiliary magnetic layer 5 in which TCR1 is exceeded is smaller than the size of recording magnetic domain 22, the time for which this temperature region is over only a single recording magnetic domain becomes comparatively long. At both the instant shown in FIG. 29A and the instant shown in FIG. 29B, magnetic domain 21 resulting from the transfer of magnetization of recording magnetic domain 22 due to TCR1 being exceeded is entirely contained in the region above recording magnetic domain 22. As a result, the magnetization can be reliably transferred from recording magnetic domain 22 to first auxiliary magnetic layer 5. The above reason applies even in the case of a perpendicularly magnetizable film wherein the first auxiliary magnetic layer 5 is at room temperature or above. That is, it is beneficial to arrange for transfer to be effected such that the magnetic domain that is transferred to the first auxiliary magnetic layer from the magneto-optical recording layer is reduced in size, even when a magnetic material that shows perpendicular magnetization at room temperature or above is employed as the first auxiliary magnetic layer.
Making the size of magnetic domain 21 that is transferred to first auxiliary magnetic layer 5 smaller than that of recording magnetic domain 22 of magneto-optical recording layer 6 is also beneficial for the following reason. A recording magnetic domain S having ↓ directed magnetization is present adjacent recording magnetic domain 22 having ↑ directed magnetization. However, since first auxiliary magnetic layer 5 has in-plane magnetization within the range indicated by region W of FIG. 6, the exchange coupling force from the ↓ directed magnetic domain S of magneto-optical recording layer 6 extending to second auxiliary magnetic layer 4 is cut off by this in-plane magnetization. Consequently the in-plane magnetization of first auxiliary magnetic layer 5 effectively acts to magnify magnetic domain 23. Thus, if the size of the magnetic domain of first auxiliary magnetic layer 5 is smaller than the size of recording magnetic domain 22, the effect of this in-plane magnetization of first auxiliary magnetic layer 5 in cutting of f the exchange coupling force from the ↓ directed magnetic domain S of magneto-optical recording layer 6 extending to second auxiliary magnetic layer 4 is further increased, thereby further facilitating the magnification of magnetic domain 22 (↑ directed magnetization).
In order to make the size of the magnetic domain of first auxiliary magnetic layer 5 smaller than the size of recording magnetic domain 22, as shown in FIG. 6, the laser power and TCR1 of first auxiliary magnetic layer 5 may be adjusted such that the temperature region where the TCR1 of first auxiliary magnetic layer 5 is exceeded is smaller than the size (width) of recording magnetic domain 22 of magneto-optical recording layer 6. In the example shown in FIG. 6, furthermore, the laser power and TCR2 of second auxiliary magnetic layer 4 are adjusted such that the temperature region where the TCR2 of second auxiliary magnetic layer 4 is exceeded is larger than the size (width) of recording magnetic domain 22. Consequently, on reproduction, recording magnetic domain 22 of magneto-optical recording layer 6 is transferred as magnetic domain 21 of first auxiliary magnetic layer 5 with its size reduced and furthermore magnetic domain 21 is transferred to second auxiliary magnetic layer 4 as magnetic domain 23 with its size increased.
It should be noted that the fact that the size of the magnetic domain 21 that is transferred to first auxiliary magnetic layer 5 is smaller than the size of recording magnetic domain 22 of magneto-optical recording layer 6 can be verified for example by the following method: substrate 1 is removed from a magneto-optical recording medium shown in FIG. 1 on which information has been recorded and dielectric layer (film) 3 and secondary auxiliary magnetic layer 4 are removed by for example sputtering; the surface of the first auxiliary magnetic layer is then heated to reproduction temperature and examined using an optical microscope etc.
The benefit in terms of reproduction signal amplification which is obtained by magnification of magnetic domain 23 of second auxiliary magnetic layer 4 is a maximum when the transferred magnetic domain in second auxiliary magnetic layer 4 is magnified to the diameter of the reproducing light beam spot. In this condition the magnitude of the reproduction signal depends on the figure of merit such as of the Kerr effect of the second auxiliary magnetic layer 4 and the reproducing light beam, irrespective of the size or configuration of recording magnetic domain 22 of magneto-optical recording layer 6. After the region of the magneto-optical recording medium from which the information has been reproduced has moved past the spot of the reproducing laser beam, its temperature falls to less than TCR2, with the result that the perpendicular magnetization of the first and second auxiliary magnetic layers returns to in-plane magnetization i.e. the state of FIG. 2A is again produced. During the reproduction action as described above, the reproducing laser beam power is adjusted such that the maximum temperature reached by the magneto-optical recording medium is lower than the Curie temperature TC0 of magneto-optical recording layer 6, so the magnetic information recorded in magneto-optical recording layer 6 is not affected by the reproducing light beam.
With the first aspect of the present invention, the thickness of the first auxiliary magnetic layer must be equal to or more than the thickness of the magnetic wall of the first auxiliary magnetic layer. As shown in FIGS. 2A, B and FIG. 6, when the critical temperature TCR1 is exceeded, the magnetization of the first auxiliary magnetic layer 5 makes a transition from in-plane magnetization to perpendicular magnetization. In order to make this transition possible, the magnetic spin in the magnetic wall between magnetic domain 21 of first auxiliary magnetic layer 5 and the in-plane magnetized magnetic domain of first auxiliary magnetic layer 5 adjacent magnetic domain 21 (hereinbelow referred to as the magnetic wall of the first auxiliary magnetic layer) must be twisted through 90xc2x0. Furthermore it is necessary that in region W only the first magnetic layer should consist of in-plane magnetizable magnetic film, buffering the spin of the magneto-optical recording layer 6 and second auxiliary magnetic layer. Consequently, in order to allow the transition between in-plane magnetization and perpendicular magnetization in first auxiliary magnetic layer 5, the thickness of the first auxiliary magnetic layer must be, at the very least, greater than the thickness of the magnetic wall of first auxiliary magnetic layer 5.
The thickness of the magnetic wall can be measured by for example a process as described below using the Hall effect. First auxiliary magnetic layer 5, second auxiliary magnetic layer 4 and magneto-optical recording film 6 are magnetized in one direction and the Hall voltage (V2) is then measured. Further, if we take the Hall resistance and film thickness of the first auxiliary magnetic layer 5, second auxiliary magnetic layer 4 and magneto-optical recording film 6 as being respectively xcfx811, xcfx812, xcfx813, and t1, t2 and t3, the Hall voltage (V3) when there is no interface magnetic wall can be found by the following expression: V3=Ixc3x97, (t1xcfx811+t2xcfx812+t3xcfx813)/(t1+t2+t3)2 (where, in this expression, I is the current flowing into the film (layer)). Consequently, the difference V4 of the absolute value of the voltage including the interface magnetic wall |V1xe2x88x92V2| and 2V3 indicates the thickness of the interface wall.
The magnetic spin state, which indicates the Hall voltage V4, can be estimated using the exchange stiffness constant, the perpendicular magnetically anisotropic energy constant and the saturation magnetization of the respective layers. Such a method of calculating the extent of the interface magnetic wall is set out in R. Malmhall, et al., Proceedings of Optical Data Storage 1993 p204-213: this document may be referenced. In the present invention, it is desirable that the thickness of the first auxiliary magnetic layer should be at least equal to the thickness of the magnetic wall measured by the measurement method using the Hall effect described above. For example in the case where the magnetic material of auxiliary magnetic layer 5 is GdFeCo-based, for example GdXFeYCoZ (20xe2x89xa6Xxe2x89xa635, 50xe2x89xa6Yxe2x89xa6100, 0xe2x89xa6Zxe2x89xa650), the thickness of the magnetic wall calculated by the above method of calculation is calculated to be about 50 nm. Consequently, in the case where the first auxiliary magnetic layer is GdXFeYCoZ (20xe2x89xa6Xxe2x89xa635, 50xe2x89xa6Yxe2x89xa6100, 0xe2x89xa6Zxe2x89xa650), the thickness of the magnetic layer is preferably at least 50 nm.
The magnetic wall thickness as above depends on the type and composition of the magnetic material, but if magnetic material as used in the magnetic layer of the magneto-optical recording medium is employed, in general a minimum thickness of 10 nm is necessary. Consequently, in a second aspect of the present invention, it is desirable that the thickness of the first auxiliary magnetic layer should exceed 10 nm.
As an upper limit on the first auxiliary magnetic layer, it is desirable that its thickness should not be greater than 100 nm, due to limitations on the power of the semiconductor laser which constitutes the light source for reproduction. Consequently, it is desirable that the thickness t of the first auxiliary magnetic layer should be 10 less than t less than 100 nm.
According to the third aspect of this invention, there is provided a method of reproducing a magneto-optical recording medium in which a recorded signal is reproduced by irradiating a magneto-optical recording medium having a magneto-optical recording layer with a reproducing light beam and detecting the magnitude of the magneto-optical effect, characterized in that, a magneto-optical recording medium as set out in the first or the second aspects of the present invention is employed as the magneto-optical recording medium and the recording signal is reproduced by irradiating the magneto-optical recording medium with a reproducing light beam which is pulse-modulated in accordance with a reproducing clock.
According to the fourth aspect of this invention, there is provided a method of reproducing a magneto-optical recording medium in which a recorded signal is reproduced by irradiating a magneto-optical recording medium having a magneto-optical recording layer with a reproducing light beam and detecting the magnitude of the magneto-optical effect, characterized in that, magneto-optical recording medium as set out in the first or the second aspect of the present invention is employed as the magneto-optical recording medium and the recording signal is reproduced by applying to the magneto-optical recording medium an external magnetic field which is pulse-modulated in accordance with a reproducing clock.
Further in a method of reproduction according to the present invention, a recorded signal is reproduced by applying to a magneto-optical recording medium an external magnetic field that is pulse-modulated in accordance with a reproduction clock while irradiating it with a reproducing light beam field that is pulse-modulated in accordance with a reproduction clock.
Further in accordance with a fifth aspect of the present invention, there is provided a reproducing apparatus for magneto-optical recording media that is suitable for performing the reproduction method of the third aspect of the present invention. This reproducing apparatus is provided with an optical head that directs a reproducing light beam onto the magneto-optical recording medium; a clock generating unit for generating a reproducing clock; and a control unit for controlling the optical head so as to produce pulse modulation of the reproducing light beam in accordance with the reproducing clock.
In accordance with a sixth aspect of the present invention, there is provided a reproducing apparatus for magneto-optical recording media that is suitable for performing the reproduction method of the fourth aspect of the present invention. This reproducing apparatus comprises: a magnetic head that applies a reproducing magnetic field to the magneto-optical recording medium; an optical head that irradiates the magneto-optical recording medium with a reproducing light beam; a clock generating unit for generating a reproducing clock; and a control unit for controlling at least one of the magnetic head and optical head in accordance with the reproducing clock in order to pulse-modulate at least one of the reproducing magnetic field and the reproducing light beam. The reproducing apparatus of the sixth aspect may further comprise: an optical head drive unit; a first synchronisation signal generating circuit for generating a first synchronisation signal for pulse-modulating the reproducing light beam in accordance with the reproducing clock; a magnetic head drive unit; and a second synchronisation signal generating circuit for generating a second synchronisation signal for pulse-modulating the reproducing magnetic field in accordance with the reproducing clock; the magneto-optical recording medium being irradiated by the reproducing light beam which is pulse-modulated by the optical head drive unit being controlled by the first synchronisation signal and a magnetic field that is pulse-modulated by the magnetic head drive unit being controlled by the second synchronisation signal being applied to the magneto-optical recording medium.
In the reproducing method and reproducing apparatus of the present invention, the reproducing clock can be generated from a signal detected by the optical head (internal clock) or can be generated from a signal detected from pits, fine clock marks or wobble-shaped grooves formed in the magneto-optical recording medium (external clock). Also, in the reproducing apparatus of the present invention, information may be recorded by controlling the optical head and/or the magnetic head.